The present invention relates generally to lithography, and in particular, to zone plate array lithography.
In a zone plate array lithography system, an array of diffractive lenses such as Fresnel zone plates may be used to form an array of tightly focused spots on a photosensitive layer that is on top of a substrate. For example, U.S. Pat. No. 5,900,637, the disclosure of which is hereby incorporated by reference, discloses a mask-less lithography system and method that employs a multiplexed array of Fresnel zone plates. The light incident on each diffractive lens may be controlled, for example, by one pixel of a spatial light modulator. The spatial light modulator for use in such a system should provide a high refresh rate, be able to operate at short wavelengths such as under 200 nm, and be able to perform gray-scaling or intensity modulation in real time.
One commercially available spatial light modulator that may satisfy the above requirements is the grating light valve (GLV) spatial light modulator made by Silicon Light Machines of Sunnyvale Calif. The GLV consists of a linear array of pixels, and each pixel consists of six metallic ribbons that form a diffraction grating. Alternate ribbons may be moved by electrostatic actuation to provide either a reflective surface or a grating.
The lithographic resolution, however, of such a system may be limited by the contrast of the aerial image. The image contrast is dependent on the printed pattern. The optical performance may be quantified by calculating the aerial image contrast of a dense grating as a function of the half-pitch of the grating. The image contrast, K is defined as:
                    K        =                                            I              max                        -                          I              min                                                          I              max                        +                          I              min                                                          (        1        )            where Imax and Imin are defined as the maximum and minimum intensities of an illumination signal that may be employed to provide a desired pattern. For example, as shown in FIG. 1, a desired pattern 10 that includes alternating imaged regions (as shown at 12) and non-imaged regions may be created using an illumination signal 14. Note that the pitch p of the desired grating pattern 10 corresponds to the pitch p of the illumination signal 14.
The intensity profile of the illumination signal 14, however, results in an imaging pattern on a photoresist layer 16 when imaged by illumination source signals 18 (again having a pitch p). The photoresist layer 16 is supported by a wafer 20, and includes marked regions in the photoresist layer that have been exposed by the sources 18. After exposure, the photoresist layer is developed, and the marked regions are removed, leaving exposed portions 22 of the underlying wafer 20. Efforts to increase resolution (e.g., decrease the pitch p), however, may result in a degradation in image contrast, due at least in part to the intensity profile of the illumination signal 14.
In particular, the aerial images for gratings of different periods may be simulated assuming a zone plate array lithography system of numerical aperture (NA)=0.7 and λ=400 nm. The cross-section through each grating may then be averaged over several line-scans, and the image contrast may be calculated using Equation (1) above. The image contrast may be plotted as a function of k1, where k1 is a measure of the lithographic resolution (normalized to the wavelength and NA), and is given by:
                              k          1                =                              p            2                    ⁢                                    N              ⁢                                                          ⁢              A                        λ                                              (        2        )            where NA is the numerical aperture and λ is the exposing wavelength. For example, a system of the prior art may provide that k1=0.32, which corresponds to an image contrast of about 18%. As the pitch p becomes smaller, the image contrast will be negatively affected, due in part, to the spatial extent of each illumination source 18.
Contrast enhanced lithography may be employed in an effort to improve image contrast. In particular, as shown in FIGS. 2A-2D, a contrast-enhancement material 24 that is spin coated on top of a photoresist layer 26 on a wafer 28. The contrast-enhancement material 24 may, for example, be a photo-bleachable polymer, whose absorption decreases (i.e., becomes more transparent) with increasing exposure dose. The intensity of transmitted light may be plotted as a function of time for an ideal contrast-enhanced system. Prior to exposure of the contrast-enhancement material 24, the material 24 is opaque, and almost no light passes through the material 24. After sufficient exposure by illumination beams 30, the material becomes transparent and light is transmitted in areas indicated at 32. Light is let through into the photoresist layer only where the exposure dose is high enough to bleach the contrast-enhancement material 24 completely. This increases the contrast of the image recorded in the photoresist. An antireflective coating between the photoresist and the wafer may also be employed.
As shown in FIG. 2B, illumination is able to reach defined regions 34 of the wafer 28 only in areas where the contrast-enhancement material 24 has become transparent (as shown at 32). The contrast-enhanced material 24 is then removed as shown in FIG. 2C using a suitable medium in (such as water) in which the contrast-enhancement material 24 will dissolve. The defined regions 34 are then removed through photoresist development, leaving openings 36 in the photoresist layer 26 through which portions of the wafer 28 may become exposed as shown in FIG. 2D.
By employing a contrast-enhancement material and by controlling the photo-bleaching rate of the contrast-enhancement material, as well as the clearing dose of the photoresist, one may enhance the contrast of the aerial image that is recorded in the photoresist. The contrast enhancement material behaves, in essence, as a contact mask, which increases the contrast of the image recorded in the photoresist. The contrast enhancement material is removed from the resist prior to development. If the contrast-enhancement material is incompatible with the photoresist, a barrier layer is needed between the contrast-enhancement material and the photoresist. This also incurs an additional step for removal of the barrier layer after exposure. There are several commercially available contrast-enhancement materials, some of which are water-soluble.
Ideally, such contrast-enhancement materials would become bleached by the illumination signal 14 in an on/off step pattern that provides an instantaneous step at the edge of each illumination beam. Since the beams, however, provide an intensity profile as shown in FIG. 1B, the contrast-enhancement material bleaches in varying amounts with distance from the center of each illumination beam. This limits resolution. Moreover, repeated illumination near a non-imaged area may accumulate over time, and may eventually reach a threshold within the material for becoming transparent.
Contrast-enhancement may also be achieved by diluting the developer or by using thin photoresist layers, but such systems may also involve difficulties such as increased line-edge roughness, as well as difficulties with pattern transfer respectively.
There is a need therefore, for an imaging system that more efficiently and economically provides increased image contrast in mask-less lithography.